Fix for spec.to_json() for CrossingEnv (issue #396)

[mwydmuch]

Description

This PR improves CrossingEnv, so it's specs can now be stated in JSON-safe way, allowing to serialize them cleanly.
Added coverage to catch spec JSON issues across all MiniGrid environments.

Changes:

• minigrid/__init__.py: register CrossingEnv variants with obstacle_type="wall"/"lava" so spec kwargs stay JSON-safe.
• minigrid/envs/crossing.py: resolve string obstacle types to Wall/Lava, validate subclasses of WorldObj, and consistently use the resolved type for missions and grid generation.
• tests/test_envs.py: new test_spec_json_serialization ensures every tested env’s spec.to_json() round-trips through json.loads.

Fixes #396 (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)

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Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes